The present invention relates generally to the commutation of brushless DC motors in a disc drive assembly and, more particularly, to position feedback control of brushless, sensorless DC motors from standstill.
A brushless DC motor typically has a number of permanent magnets mounted on a rotor and a set of electromagnetic coils mounted on a stator. The rotor is made to rotate by energizing the coils in a specific sequence relative to the angular position of the rotor. The function of energizing the coils at specific points in the rotation is called "commutation" of the motor.
One prior method of motor commutation utilizes Hall sensors mounted on the stator for sensing the angular position of the rotor. Electronic circuits connected to the Hall sensors detect the passage of the rotor past the Hall sensors and switch the energy into a different electromagnetic coil (commutate) at that instant. This prior method of commutation of a brushless DC motor has a number of disadvantages. First, since Hall sensors are required, costs are increased. Further, reliability is frequently poor. If the Hall sensors are not accurately positioned, the motor will not operate efficiently. Other disadvantages include the Hall sensors taking up space and requiring additional wires to operate.
A second commutation method is based on back electromotive force (EMF) sensing. When the rotor rotates due to a coil being energized, an electromagnetic field is induced in the coils which are not currently energized. By sensing the resulting back EMF, the correct commutation state of the motor can be determined. Back EMF commutation has advantages in that it does not require the use of Hall sensors. However, back EMF commutation has the disadvantage of poor performance when the motor is idle or moving slowly. This is particularly a problem during start-up when no back EMF fields are present. Frequently, from standstill, a guess as to the correct commutation state is made in an attempt to cause the motor to run fast enough to produce back EMF fields. This can result in the motor moving in the wrong direction, which can damage the magnetic data heads which are in contact with the magnetic discs at start-up.
A number of techniques for determining the correct commutation state at start-up have been used to start the motor spinning so that back EMF methods can be used. These techniques are typically time consuming, requiring considerable trial and error analysis to characterize the motor. Therefore, there is a need for an improved method for determining the correct commutation state of a brushless DC motor from standstill without the use of Hall-type sensors.